1. Field of the Invention
This invention relates to a magnetostatic wave device and, more particularly, to the same which includes a ferrimagnetic substrate like, for example, a YIG thin film and is used as, for example, a filter.
2. Description of the Prior Art
FIG. 4 is a perspective view showing a magnetostatic wave resonator disclosed by Kinoshita et al. of Hitachi Co. in Ultrasonics symposium 1987, p.p. 213-216 as one example of the conventional magnetostatic wave devices forming background of the invention.
This magnetostatic wave resonator 1 includes a GGG (gadolinium, gallium, garnet) substrate 2, on one principal plane of which a YIG (Yttrium, iron, garnet) thin film 3 is formed and on the other principal plane an ground conductor 4 is formed.
On the YIG thin film 3, a rectangular input electrode 5 and a rectangular output electrode 6 are formed opposing apart in the longitudinal direction of the thin film 3. Further, on the YIG thin film 3, five narrow finger electrodes 7, 7, . . . are formed spaced narrow distance apart in the width direction of the YIG thin film 3. In this case, one end of each of the electrodes 7, 7, . . . is connected to the input electrode 5 and the other end is connected to the output electrode 6.
Further, the input electrode 5 and the ground conductor 4 are connected to respective input terminals, and the output electrode 6 and the ground conductor 4 are connected to respective output terminals.
A d.c. magnetic field is applied to the YIG thin film 3 in the direction parallel to its principal plane and parallel to the finger electrodes 7 as indicated with an arrow mark Ho of FIG. 4.
In the magnetostatic wave resonator 1, when a signal is inputted to the input terminal, a predetermined signal is outputted from the output terminal. In this case, a magnetostatic surface wave (MSSW) is propagated in the YIG thin film 3 in the width direction of the finger electrodes 7. A frequency characteristic of this magnetostatic wave resonator 1 is shown in FIG. 5.
FIG. 6 is a perspective view showing a magnetostatic wave resonator disclosed by Asao et al. of Mitsubishi Electric Co., in 1988 Institution of Electronics, Information and Communication, spring national conference report C-688 as another example of the conventional magnetostatic wave devices. In this magnetostatic wave resonator 1, a wide strip line 8 as compared with that of the resonator of FIG. 4 is formed on a YIG thin film 3. One end of the strip 8 and an ground conductor 4 are connected to respective input terminals, and the other end of the strip line 8 and the ground conductor 4 are connected to respective output terminals, and both the output terminals are short circuited. A d.c. magnetic field is applied to the YIG thin film 3 in the direction perpendicular to its principal plane as indicated with an arrow mark Ho of FIG. 6.
In the magnetostatic wave resonator 1, when a signal is inputted to the input terminal, a signal reflected at the short-circuited output terminal appears at the input terminal. At this time a magnetostatic forward volume wave (MSFVW) is propagated in the YIG thin film 3 in the width direction of the strip line 8.
However, in the magnetostatic wave resonator of FIG. 4, the maximum cut-off band elimination level, that is, the maximum attenuation levels is 22-23 dB at most and a ripple is large as shown in the frequency characteristic of FIG. 5.
Further, in the magnetostatic wave resonator of FIG. 6, the maximum cut-off band elimination level is small and the ripple is large.